Maintenance of general anaesthesia Flashcards
(37 cards)
Key components of GA
Unconsciousness
Analgesia
Muscle relaxation
Vegetative depression
Types of anaesthesia
Local/regional anaesthesia
○ Peripheral nerve blocks
○ Extradural anaesthesia
○ Splash block
○ Local infiltration/wound catheters
General anaesthesia
○ Inhalation
○ Injection
○ Or a combination of the two
Maintenance of anaesthesia
Keeping patient asleep throughout the procedures
○ w/o nociception
○ w/o movement
○ w/o awareness
How to assess level of anaesthesia
Muscle tone
Jaw tone
Palpebral reflex
Corneal reflex (if doesn’t have this it will be dead soon)
Nystagmus (common in horses)
Eye position
Lacrimation
Guedel anaesthetic staging
Awake
Stage II
Stage III
- Light (plane I)
- Medium (plane 2)
- Deep (plane 3)
Stage IV
Stage II anaesthesia
Normal breaths but irregular breath-holding
Normal size pupil
Eyeballs up
Palpebral and corneal reflexes present
Lacrimation present
Still responds to surgical stimulation
Light (plane I) stage III anaesthesia
Normal breathing with regular pattern
Pupils constricted
Eyeballs down
Palpebral may be present, corneal present
Lacrimation may be present
Likely still responds to surgical stimulus
Medium (plane 2) stage III anaesthesia
Shallow breathing - more reduced intercostally, regular pattern
Pupils slightly dilated
Eyeballs slightly further up than if light but still down
Palpebral absent, corneal present
No lacrimation
May respond to surgical stimulation
Deep (plane 3) stage III anaesthesia
Very shallow, jerky breathing - no intercostal movement
Pupils very dilated
Eyeballs up
May not have corneal reflex if very deep
No lacrimation
No response to surgical stimulation
Stage IV anaesthesia
No breathing
No iris visible - just pupil
Eyeballs up
No corneal reflex
About to die
Central eyes and palpebral reflex present during surgery
Too light
Central eye and no palpebral when under GA
Too deep
What are most inhalational anaesthetics?
Halogenated hydrocarbons
Mode of action of inhalational anaesthetics
Don’t 100% know
Interaction with lipid-membranes
Binding to specific sites in proteins
GABAa-receptors
Main effect of inhalational anaesthesia
Hypnosis - NOT analgesic
Deep enough they won’t react but still feel pain
What does MAC stand for?
Minimum alveolar concentration
What is the MAC?
The concentration of vapour needed in alveoli of the lungs to prevent movement/motor response in 50% of subjects in response to surgical (pain) stimulation
What is the MAC used for?
To assess the potency of anaesthetic inhalants (the lower the more potent) - gives a rough idea of what a specific patient needs
How can you measure the alveolar concentration of an inhalant in a clinical setting?
The end tidal concentration (e.g. et(iso) is close to the alveolar concentration
What do partial pressure changes of inhalational anaesthetic in the alveoli influence?
Partial pressure changes in the brain
Rate of induction/recovery
What do partial pressures of inhalational anaesthetics in the brain influence?
Depth of anaesthesia
Side effects of inhalational anaesthesia
Cardiovascular depression
- myocardial depression (negative inotropy)
- peripheral vasodilation
- depression of tissue autoregulation
- CNS depression (reduction in sympathetic tone)
Sensitisation of myocardium to arrhythmogenic effects of catecholamines
Respiratory depression
Trigger malignant hyperthermia
Hepatoxic (metabolised in the liver)
Nitrou oxide properties
Highly insoluble in blood -> rapid induction
2nd gas effect - takes other gases with it e.g. pulls oxygen out of brain and blood at the end of anaesthesia
Diffusion in gas filled spaces/cavities - don’t use in large animals
Effects of nitrous oxide
Analgesic (NMDA receptors, opioid system)
Anaesthetic: MAC > 100% so cannot give enough of it, can only be used in combniation
No absorption with activated charcoal
